The surface of the basement complex that underlies five counties in southwestern Indiana has been mapped by the reflection seismograph method. Seismic shot points were spaced 1 to 3 miles apart along six traverses in Gibson, Pike, Posey, Vanderburgh, and Warrick Counties. A structure map on the surface of the basement complex shows an elongate northwestward-trending depression that is as much as 22 miles in width and that has a maximum depth of 3,500 feet below the regional slope of the basement surface. Structure maps drawn on the surface of Devonian limestone, the Trenton Limestone, and the St. Peter Sandstone show a monocline over the northeast flank of this basement depression. The depression in the surface of the basement complex and the monocline shown on Paleozoic maps may be related structurally to the LaSalle Anticline.